Doctors have sped up how quickly they treat heart attack patients, but it’s not saving more lives.

Hospitals have worked hard to decrease what’s known as door-to-balloon time — the period needed to insert a small balloon inside the blocked arteries responsible for heart attacks. That time is a major factor in the way hospitals are assessed for the quality of care they deliver. The assumption is that the shorter that time, the more heart muscle is saved, and the more likely patients are to survive heart attacks.

Except that they don’t, according to the latest research published in the New England Journal of Medicine. Scientists from the University of Michigan Frankel Cardiovascular Center question the theory that when it comes to treating heart attack patients, faster is better at every step. Over the last decade, the American Heart Association and the American College of Cardiology established guidelines to help doctors limit door-to-balloon time to 90 minutes or less. In their research , the scientists of the latest study looked at a national sample of 100,000 hospital admissions for heart attack between 2005 to 2009. During this period, doctors introduced significant advances to bring down treatment time from 83 minutes to 67 minutes, and about 4.7% of the patients died. While that rate was relatively low, it was virtually unchanged from years earlier, when care was slower.

How could faster care not translate into fewer deaths? It may be that doctors’ speedier actions are putting some patients at more risk of complications by moving in too quickly with the radical intervention. “The pendulum may have swung too far,” study author Dr. Daniel Menees, an interventional cardiologist and assistant professor of internal medicine at the University of Michigan Medical School, said in a statement. “In our rush to provide treatment even faster, we may be taking patients for angioplasty who don’t need one and possibly even placing those patients at-risk.”

Door-to-balloon time is only one part of evaluating a hospital’s ability to provide quality heart attack treatment, and the results suggest that hospitals may have given it too much sway. Instead, the researchers say deaths from heart attacks may be reduced with more focus on strategies like improving education of heart attack symptoms and lowering the time it takes to transfer patients from one hospital to another if needed.